Though use of elemental mercury (Hg) is industrially and commercially regulated, since hazardous waste continues to be generated, research on hazardous waste treatment methods should go on. The U.S. environmental protection agency designates a method of treating mercury according to Land Disposal Restriction (LDR) regulations and then disposing of it in a landfill.
Mercury is an element which can not be decomposed, but it may be transformed into another form that is least harmful to the environment after disposal. Examples of mercury waste include elemental mercury, and solids, waste, soil, deposits and sludge contaminated by mercury.
Elemental mercury collected from mercury waste reacts with sulfur to produce fine mercury sulfide (HgS) powder, which has a melting point of 583.5° C. and a very low solubility of 2.0×10−49. However, since it is impossible to dispose of the powdery mercury sulfide or to store it due to risk of dispersion, it has been stored in air-tight containers or disposed of after being hardened using a solidifying agent (e.g., cement). Generally, elemental mercury reacts with a large amount of sulfur for reacting more rapidly. Conventional art uses sulfur that has an about 6-fold higher molar ratio and the same weight as elemental mercury. After reaction, a mixture of mercury sulfide (HgS) and remaining sulfur are left over. Since the remaining sulfur is difficult to separate from the mercury sulfide (HgS), in the conventional art, a mixture itself thereof was solidified with cement and then disposed of, and therefore, the waste mixture has significantly increased weight and volume. When mercury melts in a liquid, mercury sulfide (insoluble material) can be prepared using sulfur or sulfide and separated by filtering so that the mercury sulfide can be isolated for solidification. However, it is very difficult to induce direct reaction of elemental mercury which is collected from solid waste using thermal extraction and condensed, with sulfur, and then separate the elemental mercury from the sulfur for solidification.
Another method for disposal of elemental mercury is a method using sulfur polymer cement (SPC). SPC is a thermoplastic material consisting of 95 wt % elemental sulfur, 5 wt % organic modifiers, dicyclopentadiene and oligomers of cyclopentadiene. SPC melts at about 120° C. and then is solidified by cooling at below the temperature.
According to the method using SPC, firstly, SPC having a size capable of passing through a 60-mesh sieve is prepared using a ball mill and then reacts with elemental mercury. Subsequently, a large amount of the SPC reacts with the elemental mercury to form a mercury sulfide compound. Then, the compound is heated up to 120° C. Accordingly, the compound completely melts into a liquid, and then a hard solidified product is prepared by cooling the melted compound. Here, for further stabilization, a small amount of sodium sulfide nonahydrate (Na2S.9H2O) may be added. SPC is generally harder and more stable than cement, so it represents excellent physical characteristics. However, SPC is a patented material which can only be obtained under license.
Thus, development of a novel treatment method to stabilize and (or) solidify elemental mercury is a matter of urgency, considering the need for original technology and export of the same.